Ed Miliband and Ed Balls. 'The nature of the Labour threat was cast as a combination of weak leadership and fiscal laxity.' Photograph: Matt Cardy/Getty Images

The resilience of the Conservative vote in England cannot purely be a function of short-term campaign tactics. Certainly the threat of a Labour government propped up by Scottish nationalists – raised relentlessly in the final weeks – will have nudged some undecided voters back into the Tory column. Labour candidates were reporting that it was one of the only messages from any party that had “cut through” in marginal seats.

But to spook people so effectively, a scare like that has to be resonating with deeper-seated concerns. That will have been a residual wariness of putting Ed Miliband in charge that dates back to the first years of the parliament. The Conservative campaign was built on two deep foundations: a public perception that the years of economic malaise were Labour’s fault – a function of carelessness with taxpayers’ money – and greater readiness to see David Cameron as a natural prime minister. The SNP-Miliband spectre deftly animated those feelings. The nature of the threat was cast as a combination of weak leadership and fiscal laxity – “a party that would break up the country and one that would bankrupt the country.”

Not for nothing did Cameron carry around with him a copy of Liam Byrne’s notorious Treasury farewell note (“sorry, there’s no money left”) for ready brandishing in stump speeches. Tory strategists cheered when Miliband refused to show a flicker of contrition for his party’s budget management during a Question Time debate a week before polling day.

While the SNP scaremongering may have looked like a last-minute tactical gambit, the Tories were confident that it played to their core message, prepared over a year ago, that only one party offered stability and security. In uncertain economic times that seems to have been sufficient to sway large numbers of undecided voters and, crucially, to tip some Ukip-leaning people away from Farage.

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This is in part a vindication of the ferociously single-minded campaign approach championed by Lynton Crosby, Cameron’s strategist. The essential elements of the Crosby method are to eliminate discussion of issues where the party is weak (the health service, for example) with carpet-bombing of messages connected to the party’s strength: safe hands on the public purse. It helped, of course, having most newspapers amplify the Tory line, but that alone cannot account for Labour’s underperformance. Cameron and the Conservatives have polled comfortably ahead on leadership and the economy throughout this parliament. Combined, that was Labour’s biggest bruise and Crosby found a well-targeted way to punch it hard.